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6-2 Sun StorEdge 5310 NAS Troubleshooting Guide • December 2004
Checkpoints of a volume are accessed through a separate fs_online. This volume
corresponds to the virtual checkpoint volume created when checkpoints are made
enabled on a volume.
FIGURE 6-1 Physical and Logical Volume Relationship
As shown in Figure 6-1, the existence of checkpoint database distinguishes the
checkpoint volume or CFS (Checkpoint File System) from the main volume or LFS
(Live File System). The checkpoint database is the data structure that virtually holds
different versions of an LFS. It is functioning as a mapping function that maps a
virtual block address to its corresponding real address on the live file system.
The checkpoint database is a flat, sparse file with one entry per each block address in
the live file system. Each entry is an array of 16 block addresses. When the mapping
function tries to resolve a virtual block address, it first locates the corresponding
entry in the checkpoint database. In order to do this, it uses the virtual block address
as an index to the file. It then uses the cpid (stored in the corresponding file-handle
of the object) as index to the entry array to find the proper real block address.
vol1
/vol1
vol1.chkpnt
checkpoint
database
Physical volume
Logical volume